Expandable reamers and stabilizers are typically employed for enlarging subterranean boreholes. Conventionally, in drilling oil, gas, and geothermal wells, casing is installed and cemented to prevent wellbore walls from caving into the subterranean borehole while providing requisite shoring for subsequent drilling operation to achieve greater depths. Casing is also conventionally installed to isolate different formations, to prevent cross-flow of formation fluids, and to enable control of formation fluids and pressure as the borehole is drilled. To increase the depth of a previously drilled borehole, new casing is laid within and extended below the previous casing. While adding additional casing allows a borehole to reach greater depths, it has the disadvantage of narrowing the borehole. Narrowing the borehole restricts the diameter of any subsequent sections of the well because the drill bit and any further casing must pass through the existing casing. As reductions in the borehole diameter are undesirable because they limit the production flow rate of oil and gas through the borehole, it is often desirable to enlarge a subterranean borehole to provide a larger borehole diameter for installing additional casing beyond previously installed casing as well as to enable better production flow rates of hydrocarbons through the borehole.
A variety of approaches have been employed for enlarging a borehole diameter. One conventional approach used to enlarge a subterranean borehole includes using eccentric and bi-center bits. Another conventional approach used to enlarge a subterranean borehole includes employing an extended, so-called, “bottom-hole assembly” (BHA) with a pilot drill bit at the distal end thereof and a reamer assembly some distance above the pilot drill bit. This arrangement permits the use of any conventional rotary drill bit type (e.g., a rock bit or a drag bit), as the pilot bit and the extended nature of the assembly permit greater flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot drill bit so that the pilot drill bit and the following reamer will traverse the path intended for the borehole. This aspect of an extended bottom-hole assembly (BHA) is particularly significant in directional drilling.
As mentioned above, conventional expandable reamers may be used to enlarge a subterranean borehole and may include blades that are pivotably, hingedly or slidably affixed to a tubular body and actuated by force-transmitting components exposed to high pressure drilling fluid flowing within a fluid channel, such as, for example, a generally axial bore, extending through the reamer tool body. The blades in these reamers are initially retracted to permit the tool to be run through the borehole on a drill string, and, once the tool has passed beyond the end of the casing, the blades are extended so the bore diameter may be increased below the casing. The force for actuating the blades to an extended position is conventionally supplied by manipulation of a drill string to which the expandable reamer is attached, hydraulic pressure of the drilling fluid within the fluid channel of the reamer tool body, or a combination of drill string movement and hydraulic pressure. In hydraulically actuated expandable reamers, the reamer tool body is typically fabricated with features and/or components for converting the hydraulic pressure of the drilling fluid within the fluid channel into an actuating force transmitted to the reamer blades. Such reamer tool bodies require complex designs with numerous moving components, as well as numerous dynamically reciprocating fluid seals to prevent unwanted leakage of drilling fluid within the tool body. Accordingly, assembling, repairing and/or servicing such expandable reamers involves complicated, time-consuming processes that must be performed by highly trained technicians.